1. Field of the Invention
The invention relates to measurement of formation parameters while drilling in a borehole and, more particularly, to the measurement of spontaneous potentials and polarization time constants of earth formations.
2. Description of the Background
In exploration and production of petroleum products from formations within the earth, numerous techniques are used to determine the location of petroleum bearing formations along a borehole. For example, there is a well established relationship between formation conductivity and the permeability of a formation and, hence, the likelihood of obtaining petroleum from that formation. Thus, it is conventional to measure the electrical conductivity of different formations along a borehole and utilize a log of formation conductivities as a function of depth to determine where to initiate attempts to extract oil from the formations.
One of the physical characteristics of the layering of different formations such as shales, sands and clays, one upon the other within the geological strata of the earth, is that different ones of these formations have different permeabilities. An effect of the different permeabilities of contiguous formations is that they each have different filtering effects upon ionized particles within the fluids contained in the formations. Thus, earth formations tend to have different resistances to the movement of ions through the formations and, consequently, ions tend to collect along the interfaces between unlike formations. This selective filtration of ions results in differences in electric potentials due to positive ion accumulation within different formations, particularly at the interfaces at adjacent dissimilar formations. This is referred to as the spontaneous potential or self-potential of a formation and often called SP for short.
When a borehole is drilled into the earth and passes through different formations, it is well-known that the potential difference with respect to an earth reference potential at the surface will vary along the length of the borehole. It is also well-known that a log of spontaneous potentials of the formations along the length of the borehole provides additional information as to the nature of the formations along the borehole and, hence, the formations most likely to be productive of petroleum.
Conventionally, spontaneous potential logs of a borehole are prepared by removing the drill stem from the hole and lowering an insulative sonde carrying an electrode down into the borehole. Measurement is made as soon as possible after removal of the string to minimize the invasion of the formations by the fluids contained within the borehole. The electrode in the sonde is connected to the surface by an insulated conductive cable upon which the sonde is lowered. A log is prepared of the electrical potential values measured by the electrode with respect to an earth potential reference point at the surface adjacent the borehole. This SP log is correlated with other logs such as conductivity to produce a fuller picture of the structure and composition of the subterranean formations along the borehole. The two major difficulties with conventional SP logging are that the drill string must be removed from the borehole before measurement and the measurements must be made relatively quickly after string removal and prior to substantial invasion of the formations by borehole fluids.
It would be highly desirable to be able to measure spontaneous potential of each formation along a borehole while the borehole is being drilled. This is true from the standpoint of early measurement prior to invasion by borehole fluids and as a continuous real time indication of the nature and characteristics of the formations being penetrated by the drill string during the drilling operation. The essence of a measurement while drilling system is to provide to the drilling operator a continuous flow of various data which are indicative of conditions within the borehole. In this manner, a drilling operator can anticipate and avoid potentially dangerous situations such as the unexpected penetration of a high pressure formation which can result in the uncontro1led flow of high pressure combustible gasses into the borehole. The latter event is known as a "blowout". The more information which can be supplied to a drilling operator during the drilling operation, the more safely and efficiently the borehole can be drilled. Measurement of spontaneous potential of each formation during a drilling operation would thus be very desirable.
The major problem which exists with measuring spontaneous potential during a drilling operation is that the highly conductive steel drill collar essentially shorts out the small spontaneous potentials which exist along the borehole. Certain techniques have been suggested in an attempt to measure the spontaneous potential while a drill string is positioned in the borehole. For example, one technique employs two longitudinally spaced electrodes positioned on, and insulated from, the drill string. These two electrodes measure the differences in the spontaneous potential at different points along the formation wall in the borehole. However, there is no direct way in which this difference potential can be related to an absolute spontaneous potential with respect to an earth reference point at the surface.
The phenomenon of spontaneous potential within earth formations is also directly related to formation polarization. Due to the fact that ionized particles are unevenly dispersed among the layers of material comprising the formations along a borehole, there exists a certain charge polarization across the spatially separated formations. This polarization phenomenon is also indicative of the geophysical nature of the formations and, is useful for identifying zones containing fluids. Polarized regions among earth formations are separated from one another by conductive fluids, such as salt water, which act as electrolytes separating the spatially separated layers of charge. By measuring and evaluating the polarization time constants of earth formations, additional information concerning the nature and shape of the formations may be obtained. Heretofore, very few techniques have existed for evaluating these very subtle relationships among charged regions dispersed within the formations surrounding a borehole.
The system of the present invention overcomes the difficulties of the prior art in the measurement of spontaneous potential along the borehole. Additionally, the present invention provides a means for measuring the spatial distribution of polarization time constants indicative of the concentration of charged ions due to ionic filtration of the formations and provides additional information as to the geophysical structure of the formations penetrated by the borehole.